Institute for Systems Research Technical Reports
Permanent URI for this collectionhttp://hdl.handle.net/1903/4376
This archive contains a collection of reports generated by the faculty and students of the Institute for Systems Research (ISR), a permanent, interdisciplinary research unit in the A. James Clark School of Engineering at the University of Maryland. ISR-based projects are conducted through partnerships with industry and government, bringing together faculty and students from multiple academic departments and colleges across the university.
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Item I-Q TCM: Reliable Communication Over the Rayleigh Fading Channel Close to the Cutoff Rate(1996) Al-Semari, Saud A.; Fuja, Tom E.; ISRThis paper presents some trellis codes that provide high coding gain over the frequency non-selective slowly Raleigh distributed fading channel. It is shown that the use of two encoders in parallel - used to specify the in-phase and quadrature components of the transmitted signal - results in greater minimum time diversity than the conventional design in which a single encoder is used. Using this approach - which we label ﲉ-Q TCM - codes with bandwidth efficiencies of 1,2, and 3 bits/sec/Hz are described for various constraint lengths. The performance of these codes is bounded analytically and approximated via simulation; the results show a large improvement in the BER when compared with conventional TCM schemes when perfect channel state information (CSI) is available to the receiver. Indeed, when this approach is applied to channels with independent Rayleigh fading, the resulting coding gain is close to that implied by the cutoff rate limit, even for only moderately complex systems.The proposed codes are also simulated under less ideal assumptions. For instance, results for a 1 bit/sec/Hz IQ-TCM code without CSI show a significant gain over conventional coding. Finally, simulations over channels with correlated fading were undertaken; it is concluded that an interleaver span of 4v yields performance close to what is achieved with ideal interleaving.
Item Performance Analysis of Coherent TCM Systems with Diversity Reception in Slow Rayleigh Fading(1996) Al-Semari, Saud A.; Fuja, Tom E.; ISRCoherent trellis coded modulation (TCM) systems employing diversity combining are analyzed. Three different kinds of combining are considered: maximal ratio, equal gain, and selection combining. For each combining scheme, the cutoff rate parameter is derived assuming transmission over a fully- interleaved channel with flat, slow, Rayleigh fading; in addition, tight upper bounds on the pairwise error probabilities are derived. These upper bounds are expressed in product form to permit bounding of the BER via the transfer function approach. In each case it is assumed that the diversity branches are independent and that the channel state information (CSI) can be recovered perfectly.Also included is an analysis of maximal ratio combining when the diversity branches are correlated; the cutoff rate and a tight upper bound on the pairwise error probability are derived. It is shown that, with double diversity, a branch correlation coefficient as high as 0.5 results in only slight performance degradation.
Item Channel Codes That Exploit the Residual Redundancy in CELP- Encoded Speech(1996) Alajaji, Fady; Phamdo, N.; Fuja, Tom E.; ISRWe consider the problem of reliably transmitting CELP-encoded speech over noisy communication channels. Our objective is to design efficient coding/decoding schemes for the transmission of the CELP line spectral parameters (LSP's) over very noisy channels.We begin by quantifying the amount of ﲲesidual redundancy inherent in the LSP's of Federal Standard 1016 CELP. This is done by modeling the LSP's as first and second-order Markov chains. Two models for LSP generation are proposed; the first model characterizes the intra-frame correlation exhibited by the LSP's, while the second model captures both intra-frame and inter-frame correlation. By comparing the entropy rates of the models thus constructed with the CELP rates, it is shown that as many as one-third of the LSP bits in every frame of speech are redundant.
We next consider methods by which this residual redundancy can be exploited by an appropriately designed channel decoder. Before transmission, the LSP's are encoded with a forward error control (FEC) code; we consider both block (Reed- Solomon) codes and convolutional codes. Soft-decision decoders that exploit the residual redundancy in the LSP's are implemented assuming additive white Gaussian noise (AWGN) and independent Rayleigh fading environments. Simulation results employing binary phaseshift keying (BPSK) indicate coding gains of 2 to 5 dB over soft-decision decoders that do not exploit the residual redundancy.
Item Distributed Decoding of Cyclic Block Codes Using a Generalization of Majority-Logic Decoding(1992) Murad, A.H.; Fuja, Tom E.; ISROne-step majority-logic decoding is one of the simplest algorithms for decoding cyclic block codes. However, it is an effective decoding scheme for very few codes. This paper presents a generalization based on the "common-symbol decoding problem." Suppose one is given M (possibly different) codes over the same field; suppose further that the codewords share a single symbol in common. The common-symbol decoding problem is that of estimating the symbol in the common position. (This is equivalent to one-step majority logic decoding when each of the "constituent" codes is a simple parity check.) This paper formulates conditions under which this decoding is possible and presents a simple algorithm that accomplishes the same. When applied to decoding cyclic block codes, this technique yields a decoder structure ideal for parallel implementation. Furthermore, this approach frequently results in a decoder capable of correcting more errors than one-step majority-logic decoding. To demonstrate the simplicity of the resulting decoders, an example is presented.Item The Performance of Focused Error Control Codes(1990) Alajaji, Fady; Fuja, Tom E.; ISRConsider an additive noise channel with inputs and outputs in the field GF (q ) where q > 2; every time a symbol is transmitted over such a channel, there are q - 1 different errors that can occur, corresponding to the q - 1 non-zero elements that the channel can add to the transmitted symbol. In many data communication/ storage systems, there are some errors that occur much more frequently than others; however, traditional error correcting codes- designed with respect to the Hamming metric - treat each of these q - 1 errors the same. Fuja and Heegard have designed a class of codes, called focused error control codes, that offer different levels of protection against "common" and "uncommon" errors; the idea is to define the level of protection in a way based not only on the number of errors, but the kind as well. In this paper, the performance of these codes is analyzed with respect to idealized "skewed" channels as well as realistic non-binary modulation schemes. It is shown that focused codes, used in conjunction with PSK and QAM signaling, can provide more than 1.0 dB of additional coding gain when compared with Reed- Solomon codes for small blocklengths.Item The Reliability of Systems with Two Levels of Fault Tolerance: The Return of the "Birthday Surprise"(1990) Fuja, Tom E.; Yang, G.C.; ISRThis paper considers the reliability of systems that employ fault tolerance at two different hierarchical levels. Specifically, it assumes the system consists of a two-dimensional array of components. Each component is reliable as long as it has been afflicted by no more than t faults; when t + 1 faults occur in a particular component, the component ceases to be reliable. Furthermore, the system remains operative as long no more than one component in any row is unreliable. By generalizing the techniques used to analyze the well-known "birthday surprise" problem of applied probability, we derive an approximation to the average number of faults needed until the systems fails. Applications include random access memory systems with chip-level and board-level coding as well as fault-tolerant systolic arrays.